1. Field of the Invention
The present invention pertains to a titanium nitride film having a particular structure which provides a low resistivity and a smooth surface, and to the method used to create this film.
2. Brief Description of the Background Art
Titanium nitride layers have been used in semiconductor device structures as barrier layers for preventing the interdiffusion of adjacent layers of materials such as aluminum and silicon, for example. However, the resistivity of the titanium nitride, typically greater than 100 .mu..OMEGA.-cm, detracts from the overall conductivity of the layered conductive structure to which it contributes. Further, if the titanium nitride surface is rough, this rough surface is mirrored in overlying layers, in an overlying aluminum layer, for example. A rough surface on the aluminum layer makes difficult subsequent photolithographic indexing process steps necessary for formation of the overall semiconductor device structure.
U.S. Pat. No. 4,514,437 to Prem Nath, issued Apr. 30, 1985, discloses a method and apparatus for depositing thin films, such as indium tin oxide, onto substrates. The deposition comprises one step in the fabrication of electronic, semiconductor and photovoltaic devices. An electron beam is used to vaporize a source of solid material, and electromagnetic energy is used to provide an ionizable plasma from reactant gases. By passing the vaporized solid material through the plasma, it is activated prior to deposition onto a substrate. In this manner, the solid material and the reactant gases are excited to facilitate their interaction prior to the deposition of the newly formed compound onto the substrate.
U.S. Pat. No. 4,944,961 to Lu et al., issued Jul. 31, 1990, describes a process for partially ionized beam deposition of metals or metal alloys on substrates, such as semiconductor wafers. Metal vaporized from a crucible is partially ionized at the crucible exit, and the ionized vapor is drawn to the substrate by an imposed bias. Control of substrate temperature is said to allow non-conformal coverage of stepped surfaces such as trenches or vias. When higher temperatures are used, stepped surfaces are planarized. The examples given are for aluminum deposition, where the non-conformal deposition is carried out with substrate temperatures ranging between about 150.degree. C. and about 200.degree. C., and the planarized deposition is carried out with substrate temperatures ranging between about 250.degree. C. and about 350.degree. C.
U.S. Pat. No. 4,976,839 to Minoru Inoue, issued Dec. 11, 1990 discloses a titanium nitride barrier layer of 500 .ANG. to 2,000 .ANG. in thickness formed by reactive sputtering in a mixed gas including oxygen in a proportion of 1% to 5% by volume relative to the other gases, comprising an inert gas and nitrogen. The temperature of the silicon substrate during deposition of the titanium nitride barrier layer ranged between about 350.degree. C. and about 500.degree. C. during the sputtering, and the resistivity of the titanium nitride film was "less than 100 .mu..OMEGA.-cm", with no specific numbers other than the 100 .mu..OMEGA.-cm given.
S. M. Rossnagel and J. Hopwood describe a technique of combining conventional magnetron sputtering with a high density, inductively coupled rf plasma in the region between the sputtering cathode and the substrate in their 1993 article titled "Metal ion deposition from ionized magnetron sputtering discharge", published in the J. Vac. Sci. Technol. B. Vol. 12, No. 1, Jan/Feb 1994. One of the examples given is for titanium nitride film deposition using reactive sputtering, where a titanium cathode is used in combination with a plasma formed from a combination of argon and nitrogen gases. The resistivity of the films produced ranged from about 200 .mu..OMEGA.-cm to about 75 .mu..OMEGA.-cm, where higher ion energies were required to produce the lower resistivity films. The higher the ion energy, the more highly stressed the films, however. Peeling of the film was common at thicknesses over 700 .ANG., with depositions on circuit topography features delaminating upon cleaving.
U.S. patent application, Ser. No. 08/511,825 of Xu et al. filed Aug. 7, 1995, assigned to the Assignee of the present invention, and hereby incorporated by reference in its entirety, describes a method of forming a titanium nitride-comprising barrier layer which acts as a carrier layer. The carrier layer enables the filling of apertures such as vias, holes or trenches of high aspect ratio and the planarization of a conductive film deposited over the carrier layer at reduced temperatures compared to prior art methods.
A titanium nitride barrier layer is most commonly used as part of a conductive stack of materials. In order to obtain optimized functionality of this barrier layer, the layer must prevent the diffusion or migration of adjacent materials through it (it must act as a barrier); it must provide high conductivity (exhibit minimal resistivity); and, it must provide a smooth surface so that other materials in the stack will not mirror surface roughness in the titanium nitride layer, thereby making subsequent lithography difficult.
It is important for the titanium nitride (TiN) film to have low resistivity, because in a typical interconnect structure, a cross-sectional schematic of which is shown in FIGS. 1A and 1B, the TiN film may need to serve as a main conductive path for the interconnect. For example, with reference to FIG. 1A, an interconnect structure 10 typically includes: a dielectric substrate 11; a diffusion barrier layer 14; and an overlying conductive layer 12. Aluminum is the most commonly used material for conductive layer 12. Arrow 16 represents the direction of travel of electrons, the electron path, through the aluminum conductive layer 12 of interconnect 10. However, the conductivity of an aluminum conductive layer 12 can become impaired due to stress or electromigration, which creates void structures of the kind shown in FIG. 1B. The interconnect structure 20 shows the dielectric substrate 21 having a diffusion barrier layer 24, followed by aluminum layer 22. The electron path 26 through aluminum layer 22 has become impaired due to the formation of voids 23 within aluminum layer 22. When the barrier layer 24 comprises TiN which is low in resistivity, the electron path 26 can then be assisted by a second path 28 through the TiN barrier layer 24. This facilitates the performance of interconnect 20, lengthening the performance lifetime of the interconnect. In some cases, the TiN barrier layer electron path 28 may become the main path of conductivity for interconnect 20. TiN barrier layers conventionally applied using physical vapor deposition (sputtering) techniques generally have a resistivity greater than about 100 .mu.Ohm-cm.
It is important for the TiN film to be smooth (to have low surface roughness), as disclosed in U.S. patent application Ser. No. 08/511,825 of Xu et al. When the titanium nitride barrier layer is used to line a feature, such as a contact, trench, or via, a smooth surface on the titanium nitride layer facilitates the formation of the carrier layer of the kind described in Xu et al. This carrier layer facilitates aluminum filling of the feature at lower temperatures. Xu et al. were able to achieve a carrier layer surface roughness of about 15 .ANG. rms.
A reduction in resistivity to less than about 100 .mu.Ohm-cm. and an improvement in surface roughness over that previously obtained would increase the desirability of the application of titanium nitride barrier layers.